1. Field of the Invention
The present invention relates to a drive apparatus equipped with a ball screw, and an XY drive apparatus containing said drive apparatus.
2. Description of the Prior Art
FIG. 1 indicates an XY drive apparatus of the prior art. Furthermore, this XY drive apparatus is disclosed in Japanese Laid-Open Patent Publication No. 2-116442.
As shown in the drawing, said apparatus respectively has relative moving members in the form of bed 1 and moving table 2. In this case, however, bed 1 is taken to be the stationary side and moving table 2, located above said bed 1, operates as the moving side. Moving table 2 is guided by a guiding device having the constitution described below, and freely moves two-dimensionally with respect to bed 1.
Namely, two track rails 4 are fixed in parallel on the upper surface of bed 1. Another two track rails 5 are attached in parallel on the lower surface of moving table 2 so as to transverse said track rails 4. Sliders 7 are respectively arranged between the mutually intersecting portions of these upper and lower track rails 4 and 5. Each of said sliders 7 is engaged to freely move relative to each of both track rails 4 and 5. As a result of having said constitution, moving table 2 freely moves two-dimensionally in the X and Y directions.
Next, the following provides an explanation of the driving device that drives the above-mentioned moving table 2.
As shown in the drawing, motors (rotating type) 11 and 12 are mounted on bed 1 and moving table 2 via brackets 11a and 12a so that the central axis of rotation of each coincides with the above-mentioned X and Y directions. Each of said motors contains an internal ball nut (not shown) so that it integrally rotates with its rotor core (not shown), and are provided with threaded shafts 14 and 15, respectively, onto which these ball nuts are screwed. One end of each of said threaded shafts 14 and 15 is coupled to the above-mentioned sliders 7 via coupling plates 17 and 18. As a result of the driving device being composed in this manner, in this constitution, moving table 2 operates by each of motors 11 and 12 suitably rotating.
In the apparatus like that described above, the feeding positional accuracy of the moving table is affected not only by the mechanical accuracy of the ball nuts, threaded shafts, track rails and so forth, but also by the number of divisions of the encoder that detects the amount of rotation of the ball nuts or threaded shafts. The number of divisions refers to the resolution by which the amount of rotation of the ball nuts or threaded shafts is detected. Thus, the use of an encoder with a large number of divisions results in higher resolution of feeding control of the moving table, thereby improving feeding positional accuracy. However, encoders having a large number of divisions are expensive, thus resulting in increased costs.